Diametral control of rolled rings

ABSTRACT

The invention relates to the roll forming of cylindrical workpieces. When the initial workpiece has a size which is likely to vary from the specified standard size by small amount, i.e., a commercially acceptable tolerance, this leads to problems because if the initial workpiece is too small the rolled ring may not have the correct profile and if the initial workpiece is too large it may fracture during rolling. It has been found according to the invention, that by determining the variation of the size of the initial workpiece from the standard size, one can adjust either the depth of rolling or the dwell time in dependence upon this variation to give rolled rings of constant diametrical size.

United States Patent [191 Andriessen DIAMETRAL CONTROL OF ROLLED RKNGS [75] Inventor: Ronald Carl Andriessen,

Cheltenham, England [73] Assignee: Formtlo Limited, Gloucestershire,

England [22] Filed: Oct. 5, 1972 [21] Appl. No.: 296,367

[30] Foreign Application Priority Data Oct. 7, 1971 Great Britain 46833/7l 52 user. 72/108, 72/30 [51] Int. Cl. B2111 M06 [58] Field of Search..... 72/30, 80, 102, 108, 365, 72/366; 29/l48.4 R

[56] References Cited UNITED STATES PATENTS 3,230,606 l/l966 Tadashi Saito et al. 29/1484 3,528,271 9/1970 DiPonio 72/108 FOREIGN PATENTS OR APPLICATIONS 1,246,549 9/1971 Great Britain 72/108 [5 7] ABSTRACT The invention relates to the, roll forming of cylindrical workpieces. When the initial workpiece has a size which is likely to vary from the specified standard size by small amount, i.e., a commercially acceptable tolerance, this leads to problems because if the initial workpiece is too small the rolled ring may not have the correct profile and if the initial workpiece is too large it may fracture during telling it has been found according to the invention, that by determining the variation of the size of the initial workpiece from the standard size, one can adjust either the depth of rolling or the dwell time in dependence upon this variation to give rolled rings of constant diametrical size.

12 Claims, 3 Drawing Figures PATENTED OUT 8 I974 SHEET 2 0F 3 wig gi $33 a 1 DIAMETRAL CONTROL OF ROLLED RINGS This invention relates to the rolling of rings and in particular relates to the rolling of inner and outer bearing races from cylindrical workpieces.

BACKGROUND TO THE INVENTION Initially cylindrical preformed workpieces or blanks can be rolled and squeezed between a pair of forming members to form shaped rings, the forming rolls impressing or squeezing into the workpieces a shape corresponding to the profile of the forming members. We find that if the rolled rings are to have a consistent high degree of diametral accuracy as is required for their use as inner and outer bearings rings, the workpieces must have an accurate initial volume. Minor variations between workpieces can cause large variations in size between the rolled rings, and can also lead to incompletely formed rings or fracture of the ring. One way of maintaining this accuracy is to produce workpieces having very small size tolerances but this is naturally rather expensive. I I

It is therefore an object of this invention to give rolled shaped rings having constant diametral size starting from workpieces having normal commercial size tolerances.

BRIEF DESCRIPTION OF THE INVENTION According to the invention there is provided a method of roll forming a cylindrical workpiece to a desired shape from a cylindrical workpiece having dimensions within a tolerance range, in which the variations in the dimensions of the cylindrical workpiece from a nominally sized workpiece are determined and the workpiece is then roll formed to its desired shape by being squeezed between at least two rotating forming members which advance relatively towards one another to a fully advanced position, i.e., depth of roll, to

impress the desired shape into the workpiece followed by maintaining the forming members at their fully advanced position to complete the roll forming for a period of time known as the dwell time, and in which extent of advancement of the forming members or the dwell time is varied from the standard required for the nominally sized workpiece in dependence upon the variations in the dimensions of the cylindrical workpiece from the nominally sized workpiece so as to produce a final rolled workpiece whose diameter is substantially identical with that of a nominally sized work piece subjected to the standard extent of advancement of the forming members or dwell time.

The variation from standard advancement of the forming members or dwell time for a particular forming operation and workpiece material, is determined from the algebraic sum relating to the variation of wall thickness, width and mean diameter from the normally sized workpiece, i.e., theworkpiece whose dimensions are exactly those required and whose size variations from those dimensions are zero. The variation from standard rolling conditions determined by the above algebraic sum is dependent upon a number of factors such as the particular configuration to be rolled into the workpiece, the actual dimensions and material of the workpiece and the rolling conditions. However, for a particular rolling operation it is readily possible by simple trial and experiment to select the way in which the variation from the standard rolling conditions is to vary in accordance with the above algebraic sum.

Volumetric variations between workpieces can be accommodated during the rolling by allowing the wall thickness of the rolled ring to vary. The blank volume variations can be split into three categories:

a. wall thickness,

b. width,

0. means diameter.

This is necessary because a certain volume increase in width has a different diametrical growth effect from an equivalent increase in volume because of an increase in the wall thickness or mean diameter.

Wall thickness and width variations give a linear rolled variation of a particular ratio for a given ring. The mean diameter variation can always be taken as a 1:1 ratio since this exists in the blank before it is rolled. It can, therefore, be said that: Rolled diameter without correction Rt- Vt Rw Vw Vd where Rt ratio of variation in rolled ring diameter to variation in blank wall thickness Rw =ratio of variation in rolled ring diameter to variation in blank width Vt blank wall thickness variation from nominally size workpiece Vw blank width variation from nominally sized workpiece Vd mean diameter variation from nominally sized workpiece.

This rolled diameter can be corrected by varying the rolled wall thickness as described above either by varying the extent of advancement of the forming members or the dwell time, the dwell time beingkept constant if the extent of advancement of the forming members is varied while the extent of advancement of the forming members being kept constant if the dwell time is varied.

In the case where the dwell time is kept constant and the extent of advancementof the forming members is varied, this has the reverse effect of rolling workpieces of varying wall thickness to a constant wall thickness, i.e., if the rolled wall thickness is increased by x then the rolled mean diameter is decreased by x Rt. To increase the rolled wall thickness by x in the case where an inner bearing ring is to be rolled by being rotatably mounted on a mandrel and a pair of opposed forming rolls squeeze the workpiece between them, the forming rolls must be prevented from advancing the standard amount for a normally sized ring by an amount of 2 x since diametrically opposite sides of the ring are being rolled simultaneously. The variation in forming roll advancement from the standard will be: 2(Rt' Vt Rw' Vw Vd)/Rt Turning to a specific example to illustrate the point, if:

Vt 0.0016 inch Vw 00021 inch Vd= 0.0015 inch then the forming rolls advance must be varied from the standard by [2 (-6 X 0.0016) (2 X 0.002l) 0.0015 ]/6 0.0013 inch This means that the gap between the forming rollsmust be set 0.0013 inches less, at the end of the roll head stroke, than the standard gap for a normally sized ring.

A simple manner of varying the extent to which the forming members can advance relatively to one another is to provide them with an adjustable stop which is attached to the head carrying the forming member. The position of this stop can be adjusted automatically by, for example, threading the stop onto a threading on the head and rotating it to adjust its position by means of a stepping motor. This has the additional advantage that the dimensions of the workpiece can be measured automatically, the required movement of the stop computed automatically and the stop set automatically. In the case of the example illustrated above, the stop on each head would have needed to be moved out by 0.0013 inch so as to reduce the final gap. If on the other hand the adjustment figure had been positive, then the stops would have had to have been moved in the required distance so as to increase the final gap.

The example noted above related to the preparation of a ring shaped between opposed forming rolls and having a profile impressed in its outer facing surface, e.g., an inner bearing ring. The invention is however, equally applicable to the preparation of shaped rings having a profile impressed in their inner facing cylindrical surfaces, e.g., an outer bearing ring obtained by squeezing the workpiece between an inner forming roll and an outer ring die. In both cases the squeezing force exerted by the forming members is preferably provided by hydraulic pressure acting on pistons connected to the forming rolls. In the latter case the correction to be applied to the extent of advancement of the forming roll and ring die towards one another is given by the expression:

which is of course half that for the process noted above because only one rolled wall thickness is being rolled.

By following the invention it is found that one can maintain highly accurate control in the manufacture of shaped rolled rings. Thus, for example, using workpieces having a tolerance of i 0.003 such in all directions, one can form rolled rings having commercially acceptable diametral tolerances of i 0.003 inch or alternatively if the wall thickness of the blank is maintained at say 1 0.0015 inch then, the blank diametral tolerance can be increased to i- 0.005 inch without affecting the rolled diametral tolerance of i 0.003 inch. This difference is due to the fact that the wall thickness factor influences the rolled diameter considerably more than either the mean diameter or width of the blank and this may in turn influence the manner in which the blank is produced. Without the correction provided by the invention the rolled diametral tolerances would be about i- .025 inch with the result that the smaller rings would not form fully and the larger rings could fracture.

Instead of adjusting the depth of roll as discussed above one can alternatively vary the dwell time and keep the depth of roll constant to achieve the same final result, namely consistently sized and shaped rolled rings. This is possible because, as the dwell time is increased, the finished rolled ring diameter is decreased in the case of rolling a profile into the outerfacing surface of a workpiece by rolling it between two opposed forming rolls and is increased in the case of rolling a profile into the inner facing surface of a workpiece by rolling it between an inner forming roll and an outer ring die. As with the adjustment of rolling depth, however, one can vary the dwell time in accordance with the algebraic sum relating to variations of wall thickness, width and mean diameter from those of normally sized workpieces, the adjustment being constant for a particular working operation, for example, a particular configuration to be rolled and dimensions and material of the workpiece. The required adjustment can however, readily be determined for other rolling parame ters.

The dwell time can be accurately controlled by utilising a pressure switch in the hydraulic supply system for advancing the forming members. This pressure switch can be actuated when the roll head reaches a fixed stop and the pressure rapidly rises to its pre-set pressure. The pressure switch can actuate an adjustable timer which can, either manually or automatically, be set according to the parameters of the workpiece to give the desired dwell time before the forming member retracts and rolling is complete. The pressure exerted on the workpiece during the dwell time is of course kept constant by the fixed stop.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be illustrated, by way of ex ample, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic elevation of a roll forming machine arranged to operate according to the invention, the right hand forming roll illustrating the roll forming controlled by variation in the depth of rolling and the left hand forming roll illustrating the roll forming controlled by variation in dwell time;

FIG. 2 is a graph illustrating the variation of depth of rolling required against the volume variation factor of the initial workpiece for one particular rolling operation to give a consistent final rolled ring; and

FIG. 3 is a graph illustrating the variation of dwell time required against the volume variation factor of the initial workpiece to one particular rolling operation to give a consistent final rolled ring.

DESCRIPTION OF THE PREFERRED EMODIMENTS The roll forming machine 10 shown in FIG. 1 has a pair of diametrically opposed forming rolls l2 and 14. These are rotatably mounted in yokes 16. The latter are fixed to the piston rods 18 of hydraulic rams 20 supplied with hydraulic fluid through the lines 22.

A workpiece 24 to be shaped by the roll forming is rotatably mounted on a mandrel 26 between the forming rolls l2 and 14. A pair of diametrically opposed growth control rolls 28 are spaced around the workpiece 24 at an angle of with respect to the forming rolls.

In order to shape the workpiece 24 it is first mounted on the mandrel 26 and then hydraulic fluid is supplied to the rams 20 causing their pistons 30 to advance the yokes 16 towards one another. At the same time the forming rolls 12 and 14 are rotated in the same direction and the advancement of yokes 16 towards one another causes the forming rolls to engage the workpiece 24 and the profile on the forming rolls l2 and 14 is impressed into the workpiece. At the same time, the two growth control rolls 28 are advanced towards one an other to prevent substantial ovality of the workpiece and consequent risk of fracture.

This operation of the machine is described fully in copending continuation in part application Ser. No. 227,799 filed 22nd Feb. 1972, to which reference is made. in that patent application is given a full description of the forming rolls 12 and M and the operation of the growth control rolls 28.

The hydraulic rams cause the growth control rolls i2 and 141 to advance to a predetermined point to give a particular depth of rolling. The extent of advancement of the rolls l2 and lid is limited by either a fixed stop 34 as shown on the end of the left hand piston rod in or an adjustable stop 36 as shown on the right hand end of the piston rod lb. When these stops engage the cylinder of the hydraulic ram further advance of the forming rolls l2 and M is prevented and the required depth of rolling has then been achieved.

Once the forming rolls have advanced to this point they are maintained and continue to roll the workpiece 24 for a period of time known as the dwell time after which the hydraulic rams 20 retract taking with them the forming rolls 12 and M and at the same time the growth control rolls 2% also retract. The shaped workpiece 24 can then be removed from the mandrel 26.

Because the dimensions of the initial workpiece 26 may vary within the tolerances allowed, if steps are not taken to take account of the variation from'the standard size, there is either a risk of fracture of the workpiece or a risk that the workpiece will not be given the exact desired profile. Therefore, according to the in vention one does ensure that the final diametrical size of the workpiece is accurate and this is achieved in one of two ways.

Before the workpiece 24 is placed on the mandrel its size is very accurately measured. This can be achieved manually but is preferably achieved automatically in a device 40 which measures its size and computes the volume variation factor from the standard size of the workpiece. The workpiece is then placed on the mandrel and rolling is effected as noted above.

To achieve the required final diametrical size the piston rods 16 can both be provided with adjustable stops 36 as shown on the right hand side of FIG. ll. These stops are threaded onto the ends of the piston rods lb and when the device 4M) has computed the volume variation factor it transmits a signal via the line d2 to a stepping motor M which rotates the adjustable stop 36 until it reaches the correct position to adjust the depth of roll to that required to give the desired constant final diameter. As explained above this may involve threading the stop 36 in or out relatively to the piston 36b.

in the alternative procedure according to the invention the device d ll computes the appropriate dwell time. In this case both piston rods 18 are provided with a fixed stop 34 as shown in the left hand side of FIG. 1. Hydraulic fluid is supplied via the line 22 to the cylinders and when the fixed stops 34 abut the ends of the cylinders of the rams 20, the hydraulic pressure builds up and is transmitted via the line 48 to a pressure sensitive switch 50 to actuate it. Actuation of this switch initiates the operation of a timer 52 which has previously been preset by the device 40 to the desired dwell time. At the end of the desired dwell time the timer 52 initiates the retraction of the forming rolls l2 and 114.

By operating in either of these ways according to the invention one can ensure that the final overall diametrical size of the roll workpiece 24 is kept at a constant figure even though the size of the initial workpiece may vary within the commercial tolerances of cylindrical workpieces prepared from commercially available cylindrical material.-

FIG. 2 is a graph showing the position at which the adjustable stop 36 must be set against the volume variation factor which is computed by the device M for a particular rolling operation, e.g., using a particular type of workpiece and rolling conditions. As can be seen once the volume variation factor has been computed by the device W in the way explained above the variation in the position of the stop is linear.

FIG. 3 on the other hand shown the equivalent case where the control is achieved by variation in the dwell time. Thus, once the volume variation factor has been computed by the device 40 it adjusts the timer 52 as shown in the graph of FIG. 3 which has been prepared for particular rolling conditions and a particular work piece. It will be noted in this case that the relationship between the volume variation factor and dwell time is not linear. In the particular example shown in FIG. 3 it will be seen that for a speed of rotation of the workpiece of 5.7 rps the dwell time required for the nominally sized workpiece corresponds to a dwell time of 0.78 second and when the volume variation factor is other than 0 the dwell time will be different.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other feature. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

1 claim:

l. A method of roll forming a cylindrical workpiece to a desired shape from a cylindrical workpiece having dimensions within a tolerance range, in which the variations in the dimensions of the cylindrical workpiece from a nominally sized workpiece are determined and the workpiece is then roll'formed to its desired shape by being squeezed between at least two rotating form ing members which advance relatively towards one another to a fully advanced position to impress the desired shape into the workpiece followed by maintaining the forming members at their fully advanced position to complete the roll forming for a period of time known as the dwell time, and in which extent of advancement of the forming members is varied from the standard required for the nominally sized workpiece in dependence upon the variations in the dimensions of the cylindrical workpiece from the nominally sized workpiece so as to produce a final rolled workpiece whose diameter is substantially identical with that of a nomi nally sized workpiece subjected to the 'standard extent of advancement of the forming members.

2. A method as claimed in claim 11 for the roll forming of a profile into the outer facing surface of the workpiece, in which the workpiece is rotatably mounted on a mandrel and profile is impressed. into the workpiece by a pair of diametrically opposed forming rolls.

3. A method as claimed in claim l in which the extent of advancement of the forming members is varied by one or more stops whose positions are adjusted in dependence upon the variations in the dimensions of the '7 cylindrical workpiece from the nominally sized workpiece.

4. A method as claimed in claim 3 in which the forming members are advanced relative to one another by one or more hydraulic rams, the pistons of the rams having the adjustable stops which prevent further ad vance of the forming members once the required extent of advancement to give the required diametrical size of the workpiece has been reached.

5. A method of roll forming a cylindrical workpiece to a desired shape from a cylindrical workpiece having dimensions within a tolerance range, in which the variations in the dimensions of the cylindrical workpiece from a nominally sized workpiece are determined and the workpiece is then roll formed to its desired shape by being squeezed between at least two rotating forming'members which advance relatively towards one another to a fully advanced position to impress the desired shape into the workpiece followed by maintaining the forming members at their fully advanced position to complete the roll forming for a period of time known as the dwell time, and in which the dwell time is varied from the standard required for the nominally sized workpiece in dependence upon the variations in the dimensions of the cylindrical workpiece from the'nominally sized workpiece so as' to produce a final roller workpiece whose diameter is substantially identical with that of a nominally sized workpiece subjected to the standard extent of dwell time.

6. A method as claimed in claim 5 for the roll forming of a profile into the outer facing surface of the workpiece, in which the workpiece is rotatably mounted on a mandrel and profile is impressed into the workpiece by a pair of diametrically opposed forming rolls.

7. A roll forming machine for forming cylindrical workpieces comprising:

A. at least two forming members,

B. means for advancing said forming members towards one another to advance positions to squeeze said cylindrical workpieces to a desired shape, maintaining said forming members at said advanced positions fora period known as dwell time and thereafter retracting said forming members away from one another,

C. means for measuring the dimensions of said cylindrical workpieces before forming,

D. means, coupled to said last named means, for

computing variations in the dimensions of a workpiece from a nominally sized workpiece, and

E. means, controlled by said means for computing and coupled to said means for advancing, for varying the extent of advancement of said members towards one another from the standard required for said nominally sized workpiece, said variation being fed in accordance with said computer variations in the dimensions of said workpiece,

whereby a final rolled workpiece has a final rolled diameter substantially identical with that of said nominally sized workpiece subjected to said standard extent of advancement.

8. A roll forming machine as claimed in claim '7 which has a pair of diametrically opposed forming rolls arranged to be advanced towards one another by one or more hydraulic rams to squeeze the workpiece between them.

9. A roll forming machine as claimed in claim 8 in which the hydraulic ram or rams have an adjustable stop which is arranged to be preset in dependence upon the variations in the dimensions of a workpiece from a nominally sized workpiece so as to stop advancement of the forming rolls relative to one another at the required depth of roll.

10. A roll forming machine for forming cylindrical workpieces comprising:

A. at least two forming members,

B. means for advancing said forming members towards one another to advance positions to squeeze said cylindrical workpieces to a desired shape, maintaining said forming members at said advanced positions for a period known as dwell time and thereafter returning said forming members away from one another,

C. means for measuring the dimensions of said cylindrical workpieces before forming,

D. means,-coupled to said last named means, for

computing variations in the dimensions of a workpiece from a nominally sized workpiece, and

E. means, controlled by said means for computing and coupled to said means for advancing, for varying the extent of the dwell time from the standard required for said nominally sized workpiece, said variation being set in accordance with said computer variations in the dimensions of said workpiece, whereby a final rolled workpiece has a final rolled diameter substantially identical with that of said nominally sized workpiece subjected to said standard extent of dwell time.

111. A roll forming machine as claimed in claim it) which has a pair of diametrically opposed forming rolls arranged to be advanced towards one another by one or more hydraulic rams to squeeze the workpiece between them.

12. A roll forming machine as claimed in claim 11 in which the hydraulic ram or rams have a fixed stop which limits the advancement of the forming rolls towards one another, and in which once the ram or rams reach the fixed stop a hydraulic pressure sensitive switch is arranged to actuate a timer which is arranged to control the dwell time and which is arranged to be preset in dependence'upon the variations in the dimensions of a workpiece from a nominally sized workpiece. i= =l 

1. A method of roll forming a cylindrical workpiece to a desired shape from a cylindrical workpiece having dimensions within a tolerance range, in which the variations in the dimensions of the cylindrical workpiece from a nominally sized workpiece are deTermined and the workpiece is then roll formed to its desired shape by being squeezed between at least two rotating forming members which advance relatively towards one another to a fully advanced position to impress the desired shape into the workpiece followed by maintaining the forming members at their fully advanced position to complete the roll forming for a period of time known as the dwell time, and in which extent of advancement of the forming members is varied from the standard required for the nominally sized workpiece in dependence upon the variations in the dimensions of the cylindrical workpiece from the nominally sized workpiece so as to produce a final rolled workpiece whose diameter is substantially identical with that of a nominally sized workpiece subjected to the standard extent of advancement of the forming members.
 2. A method as claimed in claim 1 for the roll forming of a profile into the outer facing surface of the workpiece, in which the workpiece is rotatably mounted on a mandrel and profile is impressed into the workpiece by a pair of diametrically opposed forming rolls.
 3. A method as claimed in claim 1 in which the extent of advancement of the forming members is varied by one or more stops whose positions are adjusted in dependence upon the variations in the dimensions of the cylindrical workpiece from the nominally sized workpiece.
 4. A method as claimed in claim 3 in which the forming members are advanced relative to one another by one or more hydraulic rams, the pistons of the rams having the adjustable stops which prevent further advance of the forming members once the required extent of advancement to give the required diametrical size of the workpiece has been reached.
 5. A method of roll forming a cylindrical workpiece to a desired shape from a cylindrical workpiece having dimensions within a tolerance range, in which the variations in the dimensions of the cylindrical workpiece from a nominally sized workpiece are determined and the workpiece is then roll formed to its desired shape by being squeezed between at least two rotating forming members which advance relatively towards one another to a fully advanced position to impress the desired shape into the workpiece followed by maintaining the forming members at their fully advanced position to complete the roll forming for a period of time known as the dwell time, and in which the dwell time is varied from the standard required for the nominally sized workpiece in dependence upon the variations in the dimensions of the cylindrical workpiece from the nominally sized workpiece so as to produce a final roller workpiece whose diameter is substantially identical with that of a nominally sized workpiece subjected to the standard extent of dwell time.
 6. A method as claimed in claim 5 for the roll forming of a profile into the outer facing surface of the workpiece, in which the workpiece is rotatably mounted on a mandrel and profile is impressed into the workpiece by a pair of diametrically opposed forming rolls.
 7. A roll forming machine for forming cylindrical workpieces comprising: A. at least two forming members, B. means for advancing said forming members towards one another to advance positions to squeeze said cylindrical workpieces to a desired shape, maintaining said forming members at said advanced positions for a period known as dwell time and thereafter retracting said forming members away from one another, C. means for measuring the dimensions of said cylindrical workpieces before forming, D. means, coupled to said last named means, for computing variations in the dimensions of a workpiece from a nominally sized workpiece, and E. means, controlled by said means for computing and coupled to said means for advancing, for varying the extent of advancement of said members towards one another from the standard required for said nominally sized workpiece, said variation being fed in accordance with said computer variations in the dimensiOns of said workpiece, whereby a final rolled workpiece has a final rolled diameter substantially identical with that of said nominally sized workpiece subjected to said standard extent of advancement.
 8. A roll forming machine as claimed in claim 7 which has a pair of diametrically opposed forming rolls arranged to be advanced towards one another by one or more hydraulic rams to squeeze the workpiece between them.
 9. A roll forming machine as claimed in claim 8 in which the hydraulic ram or rams have an adjustable stop which is arranged to be preset in dependence upon the variations in the dimensions of a workpiece from a nominally sized workpiece so as to stop advancement of the forming rolls relative to one another at the required depth of roll.
 10. A roll forming machine for forming cylindrical workpieces comprising: A. at least two forming members, B. means for advancing said forming members towards one another to advance positions to squeeze said cylindrical workpieces to a desired shape, maintaining said forming members at said advanced positions for a period known as dwell time and thereafter returning said forming members away from one another, C. means for measuring the dimensions of said cylindrical workpieces before forming, D. means, coupled to said last named means, for computing variations in the dimensions of a workpiece from a nominally sized workpiece, and E. means, controlled by said means for computing and coupled to said means for advancing, for varying the extent of the dwell time from the standard required for said nominally sized workpiece, said variation being set in accordance with said computer variations in the dimensions of said workpiece, whereby a final rolled workpiece has a final rolled diameter substantially identical with that of said nominally sized workpiece subjected to said standard extent of dwell time.
 11. A roll forming machine as claimed in claim 10 which has a pair of diametrically opposed forming rolls arranged to be advanced towards one another by one or more hydraulic rams to squeeze the workpiece between them.
 12. A roll forming machine as claimed in claim 11 in which the hydraulic ram or rams have a fixed stop which limits the advancement of the forming rolls towards one another, and in which once the ram or rams reach the fixed stop a hydraulic pressure sensitive switch is arranged to actuate a timer which is arranged to control the dwell time and which is arranged to be preset in dependence upon the variations in the dimensions of a workpiece from a nominally sized workpiece. 